1. Field of the Invention
This invention relates generally to a demand assigned multiple access protocol for use in a satellite communications system and, more particularly, to an onboard demand assigned multiple access protocol for use in connection with a processing satellite communications network that allows multiple users to efficiently use a common uplink transmission resource.
2. Discussion of the Related Art
Various communications systems, such as Ka-Band Satellite Processing Communications networks, employ processing satellites orbiting the Earth in a geosynchronous orbit. As is known in the art, processing satellites provide the transfer of data between user terminals at ground stations on the Earth. Data is transmitted to the satellite from the user terminals on a satellite uplink. The uplink signals are transmitted on various available channel slots that are separated in frequency and time. The data that is transmitted on the channel slots is usually packaged into separate ATM (asynchronous transfer mode) data cells. The ATM cells are demodulated, decoded and switched at the processing satellite, and then transmitted back to the Earth on a satellite downlink to the destination. A suitable antenna system is provided on the satellite to receive the uplink signals and transmit the downlink signals over a predetermined coverage area on the Earth.
A requirement for processing communications satellite networks is the need to regulate access to the satellite for a multitude of bursty users, for example, a set of intra-corporate, inter-site local area network (LAN) interconnections or Web browsing users (which display bursty traffic characteristics), who mutually share the uplink. In other words, a predetermined protocol is required to allow the many users of the network to gain access to the satellite on the uplink without interfering with each other. Typically, a bursty user, such as a user surfing the Web, requires a high peak transmission rate to minimize response time, but at a low average rate, i.e., duty cycle, due to a long idle period between transmissions. An example of this type of bursty transaction is Web Surfing, where the user xe2x80x9cmouse-click""sxe2x80x9d on a Web link, waits impatiently for the network to transfer the page to the local computer (high rate requirement for low delay), and then the user reads and thinks about the Web Page on the computer screen before going to the next Web link (the think-time results in a low average rate). If a dedicated uplink transmission resource (TR) were allocated to each user at the peak rate, then the average usage of the TR, i.e., the actual bits which are billable, or the actual usage, i.e., the average rate of the resource, is low because of the low duty cycle. A TR is a series of channel slots available for transmitting data on a transmission uplink path at a certain bandwidth. To solve the above-described problem, demand assigned multiple access (DAMA) protocols have been designed to improve the usage efficiency of the available uplink TRs by allowing several users to share the same uplink TR. In other words, to provide a more efficient usage of the available TRs, multiple users share the same uplink bandwidth. In general, DAMA protocols involve trading data throughput efficiency against delay, while maintaining a required quality-of-service (QoS) performance. One of the first DAMA protocols is generally known as xe2x80x9cSlotted Aloha,xe2x80x9d where the multiple users of the common TR compete for resource usage. In the Slotted Aloha technique over a fully processed satellite, any of the several users of the same TR would transmit data on the uplink whenever they desired. If another user of the same resource also transmitted data at the same time, the two uplink signals would collide in the satellite, resulting in garbled data is unusable onboard. The transmitting user detects a collision by the absence of the acknowledgment on the downlink within a predetermined time i.e., detected by timeout. In order to prevent subsequent immediate collisions by the same two users, the delay of the retransmission for each user was randomized. Although the Slotted Aloha method does increase the usage efficiency of the TRs by statistically multiplexing the uplink TR, there is a significant potential for collisions when multiple users simultaneously access the uplink, necessitating retransmission and resulting in concomitant delay by all of the users.
To limit the number of collisions, improved DAMA techniques were devised, generally referred to a reservation DAMAs. In a reservation DAMA protocol, the uplink traffic is separated into a main flow where coordinated non-contentious access is provided for user data through a reservation procedure, and a control flow which uses the contentious Slotted Aloha technique, but only for reservation requests. The reservation procedure works by specifically reserving a particular slot in the uplink TR prior to actually transmitting the desired data. By reserving particular channel slots for transmitting data on the main flow TR, the chance of a collision between users is removed. Collisions are therefore limited to the lower rate control flow resulting in a much higher usable throughput for the main flow of data. For processing satellite networks, there can be three general variations of the reservation DAMA technique, depending on where the reservations are controlled. Particularly, the reservations can be controlled at a central terrestrial location, often referred to as a network operations center (NOC), onboard the processing satellite itself, or distributed among the set of users using the TR. The first approach is usable for bent-pipe and processing satellites, and is currently in use in very small aperture terminal (VSAT) networks. However, this approach has the disadvantages that a heavy processing load to control the DAMA protocol may occur at the NOC, and two trips to the satellite are required for each reservation request, thus resulting in extra delay and increased overhead usage of the TR that is wasted bandwidth because it is not xe2x80x9cbillable.xe2x80x9d The third approach is the subject of U.S. patent application Ser. No. 09/162,514, filed Sep. 29, 1998, titled xe2x80x9cDistributed Control DAMA Protocol For Use With A Processing Communications Satellite,xe2x80x9d assigned to the assignee of the instant application.
What is needed is an uplink TR allocation technique that allows multiple bursty users to efficiently use a common uplink transmission resource in an efficient manner that avoids collisions between sharing users and conserves system resources. It is therefore an object of the present invention to provide such a technique.
In accordance with the teachings of the present invention, an onboard DAMA protocol is disclosed for use in connection with a processing satellite communications network, where multiple users are assigned to a common transmission resource as part of a sharing set. A media access controller on the satellite maintains a reservation log which identifies the uplink time slots in the resource that are currently reserved by the members of the sharing set and allows the members of the sharing set to reserve the available time slots. To reserve a time slot on the transmission resource for a non-contentious transmission of data, a user of the sharing set will transmit a reservation request to the satellite on a contentious uplink resource. The reservation request includes the amount of data to be sent. If a return message is not received, the user knows that the reservation request collided with another reservation request, and a retransmission strategy of the reservation request is employed.
When the onboard media access controller receives the reservation request, it decides whether to grant, deny or delay the request based on predetermined criteria, including the service and price class of the user, the amount of data already recently sent by the user, the number of other users with reservation requests in the queue, and the congestion state of the destination downlink port. The media access controller issues a reservation grant message to the requesting user that includes the time, frequencies and length to send the data. When the user receives the reservation grant message, it will then wait for its reserved time slot, and will then transmit its data in a non-contentious basis on the main flow TR at that time. If the user needs additional resources to transmit more data than was originally requested, it may xe2x80x9cpiggybackxe2x80x9d a next reservation request on the data transfer.
The media access controller uses a hierarchical uplink fair scheduling technique to determine when available bandwidth will be assigned to a particular user based on a packet fair queuing (PFQ) algorithm. In a first stage, a particular wholesaler is selected based on the PFQ algorithm, and then retailers associated with the selected wholesaler are selected in a particular order in a second stage based on the PFQ algorithm to determine the priority of data transfer.
The media access controller receives data from other processors in the satellite to monitor network traffic and congestion at the various destination ports to determine whether the reservation request will be granted, delayed or denied. A real-time bandwidth estimator module is provided to determine the effective or equivalent available bandwidth capacity for each downlink from the satellite. Additionally, a flow-control module is provided that senses congestion at the various destination ports serviced by the network (internal and external to the satellite). A call processor module monitors the entire transmission of data throughout the network, and provides congestion information to the flow-control module. The call processor knows the topology of the network (e.g. through the private network-to-network interface (PNNI) protocol), enabling it to re-route calls through alternate paths to a destination. An available bit-rate module calculates the proportion of excess available bandwidth based on current use, and provides this information to the media access controller.
Additional objects, advantages, and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings.